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Modern Days, Modern Ways to Identify Slack Adjusters for Heavy-Duty Trucks

Slack adjusters have a finite life expectancy.  So, if you sell heavy-duty truck parts or repair class 8 commercial trucks, you’ve no doubt faced the challenge of trying to identify a slack adjuster that has no identifiable part numbers or information.

That problem has been solved with the Diesel Parts app.

Slack Adjusters for Trucks vs. Slack Adjusters for Trailers

Slack adjusters come in many different configurations, which makes it one of the more difficult heavy-duty parts to identify if you don’t have a part number or access to a VIN lookup provided by the dealer.

Semi-truck and trailer on highway at night.

When you visit your local heavy-duty aftermarket parts store, the parts technician will probably ask you many questions. A good parts technician will first verify if it is for the steer axle or the drive axle of the truck or if it is for the trailer.

While the slack adjusters used on the drive axle of a truck and a trailer axle look very similar, they are configured differently and so this is an important thing that must be distinguished to get the correct replacement part.

Manual Slack Adjusters and Automatic Slack Adjusters

Older heavy-duty trucks and trailers will have manual slack adjusters. As their name implies, these slack adjusters need to be manually adjusted to keep air brake adjustment within safe operating specifications.

Manual Slack Adjusters vs Automatic Slack Adjusters

Newer vehicles will have automatic slack adjusters which are sometimes also referred to as self-adjusting slack adjusters. As the friction material on the brake shoes wears the slack adjuster will automatically adjust to stay within operating specifications.

Some repair technicians claim that adjusting automatic slack adjusters is sometimes required but the manufacturers of different types of automated slack adjusters, like Meritor, Haldex and Gunite publish recommended practices for maintenance, and they usually do not recommend manually adjusting truck brakes when an automatic slack adjuster is installed.

Using a Tape Measure to Identify a Slack Adjuster

Have you ever seen an experienced heavy-duty parts technician whip out a tape measure and within a few seconds they disappear behind the counter into the warehouse only to reappear moments later with the slack adjusters you’re looking for?

Identifying parts with a tape measure

It took years for most parts technicians to get enough experience to do that.

With the invention of the By Measurement Tool on Diesel Parts, now a truck driver, repair technician, or brand-new parts technician with little to no experience can easily identify slack adjusters.

How to Identify an Automatic Slack Adjuster Using Diesel Parts

Step 1: Download the app for free on your Apple or Android device or access it on your computer by visiting parts.diesellaptops.com. If this is your first-time using Diesel Parts you will need to set up your free account which is as easy as providing an email address and creating a password.

Step 2: Click on By Measurement on the left menu bar.

Step 3: Click on the Component bar in the center of the screen. Select Brakes – Automatic Slack Adjusters from the dropdown menu.

Step 4: An image of an automatic slack adjuster will appear with three things you need to identify.

  • The A measurement. Take your tape measure and measure the arm length of the slack adjuster. It requires you to measure from the center of the big gear at the bottom of the slack adjuster to the small pin at the end of the arm. Click the Arm Length box and select the measurement from the drop-down menu. The measurement will be between 5” and 6-1/2”.
  • The B Measurement. This is not really a measurement, rather the next step is to count the number of splines. The number of splines indicates whether it is coarse or fine. Click the Number of Splines box and make your selection from the drop-down menu. You will typically find 10, 24, 28, or 37 splines on an automatic slack adjuster.
  • The C Measurement. The final step is to measure the diameter of the spline. Click the Spline Diameter box and select the measurement from the drop-down menu. It will typically be either 1-14” or 1-1/2”.
  • Review the Results: The results will populate at the bottom of the page with specific notes for each part. There you will find the final bit of information you require to identify the automatic slack adjuster you need. Sometimes the arm is not straight, it may be offset or there may be an extended length collar for the clevis that makes it one part number over another.
  • Click View Part. Once you have identified the slack adjuster you need, click the view part button and you will be connected to the parts cross-reference section of Diesel Parts, and you will be able to select the brand you prefer, or choose from one of the aftermarket suppliers that make this replacement part.

Buy High-Quality Slack Adjusters

Once you have identified the slack adjuster that you need, now it is time to make a purchasing decision. The slack adjuster price will vary depending on the slack adjuster type you require. But quality will also play a major role in how much the slack adjuster costs. 

Don’t be tempted by low prices on low quality slack adjusters.

First - this is part of your brake system, so you want it to work properly.

Second, the higher-quality slack adjuster you install the longer it will last, and that will have a direct impact on lowering your cost-per-mile and TCO.

Never struggle to identify a slack adjuster, or any replacement part again, download the Diesel Parts app for free on your Apple or Android device or access it on your computer by visiting parts.diesellaptops.com.

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Jamie Irvine

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